The present invention generally relates to the art of manufacturing composite structures and particularly to a method of fabricating a removable mandrel for use in filament winding of composite parts. More specifically, the present invention relates to the making and use of Shape Memory Polymer (SMP) molds as mandrels in a continuous filament winding process or other composite part manufacturing process.
Known processes for fabricating castable composite parts are very complicated and expensive. A large portion of the complexity and expense is associated with manufacturing related molds. Nearly any part can be constructed as a composite part by various production methods such as filament winding, tape placement, overbraid, chop fiber roving, coating, painting, dripping, hand lay up, resin soaked, or other composite processing technique and curing process.
These methods for constructing composite parts such as vessels, tanks, containers, pipes, air ducts, complex parts, and other parts are well known in the prior art. When these vessels, tanks, containers, pipes, air ducts, complex parts, and other parts are manufactured using a mandrel, there is typically a problem with removing the mandrel from the finished part. It is usually not advisable to cut the part from the mandrel, as this may destroy the recently fabricated part. It is desirable to remove the mandrel from the part. It is most desirable to be able to easily and quickly remove the mandrel from the part without damaging the finished part.
Typically, the first step in producing a castable composite part is to acquire a prototype that embodies the desired features. The prototype is then given to a tool and die facility that makes molds for replicating the prototype. The molds produced may encompass an assembly of individual components in which case a series of individual molds are required. In some cases, the mold is manufactured directly from a set of drawings in a three dimensional computer readable format which embody the desired features. A mold, set of molds, or set of pieces to make a mold is often referred to as “tooling”.
Often times, castable composite parts are made with resins which require a thermal curing operation subsequent to the filament winding process. Therefore, molds for producing castable composite parts are typically made of metal, usually a type of steel, aluminum, or other durable lightweight material with a melting temperature above the anticipated curing temperature of the associated resin. Machining methods associated with producing metal molds are very expensive. Typically, a mold for casting a part with high surface definition and intricate three dimensional details is complex and expensive.
When making filament wound parts or structures, normally continuous resin soaked fiber is conventionally wound onto a mandrel in predetermined geometric patterns such as polar, helical or hoop windings, using computerized winding equipment. Creels hold the fiber and it is fed under tension. The mandrel may rotate or be passive. The orientation and thickness of the winding may be selected to match the direction and magnitude of loads in the final part or structure. Use of computer technology allows filament winding systems to produce complex shapes in addition to simple cylindrical or rectangular shapes. However, the more complex the part is the more expensive and time consuming it is to make the mold for the mandrel. Molds for parts with only one 60° angle can have dramatically higher costs than that of a simple pipe or tube.
Another continuous filament winding process, to which the present invention equally applies, is braiding in which a plurality, e.g., as many as 144 separate fiber tows are interwoven to form tubular products. Continuous filaments may also be employed in pultrusion processes, wherein a plurality of filament strands or rovings are passed under tension through a resin bath to apply a resin coating thereto, following which the filament is drawn through a pre-former, or initial forming die, which imparts a selected cross-sectional shape to the fiber array. The initially shaped fiber array is next passed through a heated die with constant cross-sectional area, by which the resin is cured, with the resulting rigid formed article being withdrawn and conveyed to a cut-off saw or other severing apparatus to form discrete product articles. The pultrusion process is conducted under continuous tension, by means of a puller or other drawing means which pulls the filament through the constituent unit step in the process systems. Additionally, other methods of manufacturing composite parts are known in the art and the present invention is not limited to the specifics processes mentioned herein.
Heretofore, the problem of removing the mandrel from the composite part has usually led to one of four solutions. The first is to sacrifice or destroy the mandrel upon removal from the finished part. This creates drastically increased costs and production times for mandrels as a new mandrel is needed for every part. The destruction of the mandrel can be accomplished in the form of a water soluble mandrel or a mandrel that is cut from the composite part. Dissolvable mandrels typically involve water soluble sand or salt formations that may be effective for large vessels but the binding agents in the mandrels are limited to relatively low temperature cures. Additionally, dimensional repeatability of sand and salt surfaces can be an extremely variable and require expensive tooling. A second solution is to use a mandrel that can be disassembled and removed from the mandrel, such as segmented mandrels. Use of such tooling to make mandrels can consume great amounts of time to install and remove and the tooling needed to form the mandrel is typically very expensive to make and can sacrifice tolerance repeatability. A third solution is to create a mandrel that remains part of the final composite part. This solution, while eliminating the need for removal of the mandrel, is still very expensive in that the mandrel must be designed and built as an integral part of the final manufacture piece. This drastically increases the overall expense of the final part. The final solution is to use inflatable mandrels that can be removed by deflating them after the part is created. Currently, this process typically involves a balloon-like mandrel that can only be used in low tolerance productions or in processes that require significantly more time and effort to make than the current invention. It is this solution that the present invention improves upon.
One known method of making inflatable mandrels is disclosed in U.S. Pat. No. 4,684,423. Using this method a rigid mandrel is prepared and supported on a rotatable axle. One or more layers of strips or rubber are applied longitudinally on the mandrel to form an enclosure. Coats of rubber solution are applied over the layers and a layer of fibers is wound over at least one of the layers. The rubber layers are then cured. The layers are cut into two parts. Cutting and splicing the mandrel structure results in an inherently weaker and less desirable mandrel. Since the area at the resulting joint is weaker than the remaining structure, the joint often fails sooner than the other portions, thus limiting the usable life of the mandrel.
Another method for fabricating an inflatable mandrel is disclosed in U.S. Pat. No. 5,259,901. Using this method a water soluble mandrel forms the base and an inflatable mandrel is constructed around the water soluble mandrel. Once the inflatable mandrel has cured, the water soluble mandrel is flushed out. The inflatable mandrel is then used to create the composite part. Once the finished part is made the inflatable mandrel is deflated and removed through an opening on the end. This process is expensive and time consuming and has the inherent problems of cost and tolerance repeatability of water soluble mandrels.
Another method for fabricating an inflatable mandrel is disclosed in U.S. Pat. No. 6,444,071. In this method, a dry three dimensional fabric layer of a given thickness is layed-up about an inflated bladder. An external vacuum/pressure bag is installed about the fabric layer. The dry fabric layer then is impregnated with a liquid soluble resin between the internal bladder and the external bag. The resin is cured to rigidify the three dimensional fabric layers to form a mandrel structure. A container then can be wound on the mandrel formed by the cured and rigidified fabric layer, and the resin subsequently is washed out to remove the fabric. The internal bladder is deflated and removed from inside the mandrel before the mandrel is used to filament wind the part. This process is also expensive and time consuming and has the inherent problems of cost and tolerance repeatability of water soluble mandrels.
Shape memory polymers (SMPs) and shape memory alloys (SMA) were first developed about 20 years ago and have been the subject of commercial development in the last 10 years. SMPs derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation. SMPs that have been preformed can be deformed to any desired shape below or above its glass transition temperature (Tg). If it is below the Tg, this process is called cold deformation. When deformation of a plastic occurs above its Tg, the process is denoted as warm deformation. In either case the SMP must remain below, or be quenched to below, the Tg while maintained in the desired thermoformed shape to “lock” in the deformation. Once the deformation is locked in, the polymer network cannot return to a relaxed state due to thermal barriers. The SMP will hold its deformed shape indefinitely until it is heated above its Tg, whereat the SMP stored mechanical strain is released and the SMP returns to its performed state.
Several known polymer types exhibit shape memory properties. Probably the best known and best researched polymer type exhibiting shape memory polymer properties is polyurethane polymers. Gordon, Proc of First Intl. Conf. Shape Memory and Superelastic Tech., 115-120 (1994) and Tobushi et al., Proc of First Intl. Conf. Shape Memory and Superelastic Tech., 109-114 (1994) exemplify studies directed to properties and application of shape memory polyurethanes. Another polymeric system based on crosslinking polyethylene homopolymer was reported by S. Ota, Radiat. Phys. Chem. 18, 81 (1981). A styrene-butadiene thermoplastic copolymer system was also described by Japan Kokai, JP 63-179955 to exhibit shape memory properties. Polyisoprene was also claimed to exhibit shape memory properties in Japan Kokai JP 62-192440. Another known polymeric system, disclosed by Kagami et al., Macromol. Rapid Communication, 17, 539-543 (1996), is the class of copolymers of stearyl acrylate and acrylic acid or methyl acrylate. Other SMP polymers known in the art includes articles formed of norbornene or dimethaneoctahydronapthalene homopolymers or copolymers, set forth in U.S. Pat. No. 4,831,094. Additionally, styrene copolymer based SMPs are disclosed in U.S. Pat. No. 6,759,481 which is incorporated herein by reference.
No known inflatable mandrel is suitable for rapid, cheap production with the ability to retain high tolerances. Polymer composite parts have the advantages of being light weight, having high specific mechanical properties, and having good corrosion resistance which make them indispensable materials. Thus a method for cheaply and quickly producing molds to manufacture these parts is needed. Furthermore, the production of complex shaped parts is still a challenge for the composite industry and the production of any complex part is both expensive and time consuming.
Therefore, there is a need in the art for a temporary, removable and reusable mandrel that provides the same high quality as other high cost metal mandrels. Additionally, there is a need for a mandrel that can quickly replicate complex parts via a mold to manufacture complex composite parts. It is these needs that the present invention satisfies.